Pneumonitis carinii is a life threatening cause of pneumonitis among patients who are immunocompromised because of malignancies, acquired immunodeficiency syndrome, congenital immunodeficiency disorders, organ transplantation or malnutrition. In spite of the fact that P. carinii has been and continues to be a well recognized cause of pneumonia in this patient group, relatively little is known about the antigenic structure of this organism or about the immunopathogenesis of P. carinii pneumonitis. Monoclonal antibodies have proven to be valuable tools in the study of cell surface structure, allowing for analysis of cell surface antigens in far greater detail than ever possible with polyclonal antisera. In order to advance our knowledge of P. carinii, monoclonal antibodies that bind to P. carinii will be produced. These antibodies will than serve as well defined reagents to (1) characterize the molecular and antigenic surface structure of P. carinii using standard immunologic techniques such as immunoblot analysis, immunoelectron microscopy and affinity chromatography, (2) develop methods for the purification of P. carinii from infected lung tissue based on specific recognition of P. carinii by monoclonal antibodies, e.g., by "panning" or through the use of the fluorescence-activated cell sorter, (3) determine if P. carinii obtained from different species share antigens recognized by the monoclonal antibodies, (4) establish the importance of humoral immunity in the control of P. carinii disease through the use of passive administration of monoclonal antibodies in the murine model of P. carinii pneumonitis, and (5) to develop more specific non-invasive techniques for the diagnosis of P. carinii disease by using the monoclonal antibodies in various forms of enzyme immunoassays to detect free P. carinii antigens.